1. Field of the Invention
This invention relates to a method of and apparatus for providing information as to the existence or position of objects otherwise than by direct vision.
Such methods and apparatus are generally known as radar and sonar and respectively make use of reflection from the objects concerned of electro-magnetic wave energy and stress wave energy, preferably in the latter case in a frequency range above audibility, i.e. supersonic.
The method to which the present invention relates (hereinafter referred to as being of the kind specified) includes the steps of radiating wave energy (herein called the transmitted signal) from a station to a field of view, receiving at the station at least part of the transmitted signal (such part herein being called the received signal) reflected from the object, carrying out a frequency modulation of the transmitted signal to produce a frequency difference between the received signal and a correspondingly frequency modulated locally generated signal in a mode such that the magnitude of the frequency difference represents the range of the object from the station, operating upon the received signal with the locally generated signal to produce a range signal of said difference frequency, and determining or sensing the angular position of the object with respect to a reference axis extending through the station and the field of view by observation of a characteristic of the received signal and which is dependent upon the angular position of the object.
The apparatus to which the invention relates (hereinafter referred to as being of the kind specified) comprises a transmitting means for transmitting wave energy (herein called the transmitted signal) from a station to a field of view, receiving means at the station for receiving at least part of the wave energy (this part herein being called the received signal) reflected from an object in the field of view, frequency modulating means for producing a frequency difference between the received signal and a locally generated signal in a mode such that the magnitude of the frequency difference is representative of the range of the object from the station, means for operating on the received signal with a locally generated signal to produce a range signal of said difference frequency, and means for producing a direction signal from the received signal representative of the angular position of the object with respect to a reference axis extending through the station into the field of view.
2. Description of the prior art
One form of apparatus of the kind specified for performing a method of the kind specified is disclosed in earlier developments of the present inventor published in U.S. Pat. Nos. 3,172,075 and 3,366,922 relating to a blind aid. In the latter, two receiving transducers on a head set provide receptivities represented by divergent medially overlapping polar lobes and signals received by these transducers are fed through channels in the receiving means providing audible difference frequency indicator signals to the left and right-hand audio transducers on the head set. The polar lobe characteristics are designed to match the characteristics of the human auditory neural system so that a blind person wearing the head set can sense the direction from which a reflection of the transmitted signal is received, and, of course, can also make a subjective determination of the range by virtue of the frequency of the difference signal.
There is difficulty, however, in a user being able to resolve two stationary objects from each other where the angular spacing of such objects within the field of view is small, unless the range difference is of the order of at least 0.4 times the range of the more remote object.
Furthermore, in both of these earlier developments the transmitted signal is subject to swept frequency modulation over a wide frequency band of almost an octave, in consequence the indicator signal has a further desirable characteristic, namely it contains frequency components producing a complex tone as a result of the transmitted signal having been reflected from different parts of an object at slightly differing ranges, and possibly having different reflective properties. This enables a blind person experienced and suitably trained in the use of the apparatus to learn to recognise certain objects, or certain surface structures of objects, but an accurate inference as to the nature of the object or surface can best be made when the signal furnished to the user is attributable to only one object in the field of view and the above identified apparatus and methods do not enable this to be accomplished.
It is nevertheless highly desirable that, initially, information by way of indicator signals should be given to the user as to the existence of all, or at least a high proportion, of the objects present in the field of view prior to the user being furnished with information such as to enable an inference to be drawn as to the nature of any one object. The prior methods and forms of apparatus do not solve the problem of how to deal with these somewhat incompatible requirements and the aim of the present invention is to do so.